Second-order rate constants for nucleophilic substitution reactions of 2,4-dinitrophenyl benzenesulfonate 1a with a series of alicyclic secondary amines in MeCN have been measured spectrophotometrically and compared with those reported previously for the corresponding reactions performed in aqueous medium to investigate the effect of medium on reactivity and reaction mechanism. The amines employed in this study are found to be more reactive in the aprotic solvent than in H2O. The reactions of 1a in MeCN result in a linear Bronsted-type plot with nuc = 0.58, which contrasts to the curved Bronsted-type plot reported previously for the corresponding reactions performed in the aqueous medium (i.e., 2 = 0.86 and 1 = 0.38). Accordingly, it has been concluded that the reaction mechanism changes from a stepwise mechanism to a concerted pathway upon changing the medium from H2O to MeCN. Reactions of Y-substituted phenyl benzenesulfonates 1a-c with piperidine in MeCN result in a linear Bronsted-type plot with lg = 1.31, indicating that expulsion of the leaving group is significantly more advanced than bond formation in the transition state. The trigonal bipyramidal intermediate (TBPy±) proposed previously for the reactions in H2O would be highly unstable in MeCN due to strong repulsion between the negative charge in TBPy± and the negative dipole end of MeCN. Thus, destabilization of TBPy± in MeCN has been concluded to change the reaction mechanism from a stepwise mechanism to a concerted pathway.